On a worldwide basis, leptospirosis is the most widespread zoonotic disease. Leptospirosis is also emerging as a significant infectious disease in urban slums, particularly in tropical regions. The infection is caused by spirochetes of the genus Leptospira, and varies in severity from mild illness to fatal hemorrhagic disease with multiple organ failure. Disease severity varies with the host species, host age and health status, and the infecting strain. Virtually all species of mammals can serve as carriers, with the Leptospira persisting in the proximal tubules of the kidney and being shed in the urine. The persistence of leptospirosis in wildlife, companion animals, livestock, and in particular peridomestic rats, results in a continuous reservoir for human infection through exposure to animal tissue or body fluids containing Leptospira, or to urine-contaminated water sources. The bacteria enter through mucous membranes or skin abrasions, then disseminate to multiple tissues and can establish persistent infection in particular sites, most importantly the proximal tubules of the kidney. In addition, widespread endothelial damage is a feature of leptospirosis. Adhesion to host cell surfaces and extracellular matrix (ECM) is critical to the pathogenesis of disease for many pathogens. This is likely to be true of Leptospira, as well, but previous investigations into adhesion of Leptospira to mammalian tissue components have focused on interactions with the ECM. This proposal will follow up on our recent findings that the bacteria bind more efficiently to the cell surface than to the ECM, and that attachment to endothelial cells triggers disruption of cell layer integrity but does not kill the cells. These results suggest the presence of additional, significant, unexplored aspects of the interactions of pathogenic leptospires with the host. The goal of this project is to employ several complementary approaches to identify Leptospira interrogans molecules involved specifically in attachment to human cell surface receptors and altering the biology of host cells. The long-term goal of this line of work is to better understand the nature and consequences of Leptospira interactions with mammalian cells, which may eventually be useful in future development of improved vaccines or therapeutic approaches.